The present disclosure relates to a photovoltaic device and a method of manufacturing the same. More particularly, the present disclosure relates to a photovoltaic device, such as a solar cell, including a multimetal semiconductor alloy layer. The present disclosure also provides a method for forming the same. The present disclosure also provides a method for forming a photovoltaic device containing a single metal semiconductor alloy layer that is formed from a multimetal layer.
A photovoltaic device is a device that converts the energy of incident photons to electromotive force (e.m.f.). Typical photovoltaic devices include solar cells, which are configured to convert the energy in the electromagnetic radiation from the Sun to electric energy. Each photon has an energy given by the formula E=hv, in which the energy E is equal to the product of the Plank constant h and the frequency v of the electromagnetic radiation associated with the photon.
A photon having energy greater than the electron binding energy of a matter can interact with the matter and free an electron from the matter. While the probability of interaction of each photon with each atom is probabilistic, a structure can be built with a sufficient thickness to cause interaction of photons with the structure with high probability. When an electron is knocked off an atom by a photon, the energy of the photon is converted to electrostatic energy and kinetic energy of the electron, the atom, and/or the crystal lattice including the atom. The electron does not need to have sufficient energy to escape the ionized atom. In the case of a material having a band structure, the electron can merely make a transition to a different band in order to absorb the energy from the photon.
The positive charge of the ionized atom can remain localized on the ionized atom, or can be shared in the lattice including the atom. When the positive charge is shared by the entire lattice, thereby becoming a non-localized charge, this charge is described as a hole in a valence band of the lattice including the atom Likewise, the electron can be non-localized and shared by all atoms in the lattice. This situation occurs in a semiconductor material, and is referred to as photogeneration of an electron-hole pair. The formation of electron-hole pairs and the efficiency of photogeneration depend on the band structure of the irradiated material and the energy of the photon. In case the irradiated material is a semiconductor material, photogeneration occurs when the energy of a photon exceeds the band gap energy, i.e., the energy difference of the conduction band and valence band.
The direction of travel of charged particles, i.e., the electrons and holes, in an irradiated material is sufficiently random (known as carrier “diffusion”). Thus, in the absence of an electric field, photogeneration of electron-hole pairs merely results in heating of the irradiated material. However, an electric field can break the spatial direction of the travel of the charged particles to harness the electrons and holes formed by photogeneration.
One exemplary method of providing an electric field is to form a p-n or p-i-n junction around the irradiated material. Due to the higher potential energy of electrons (corresponding to the lower potential energy of holes) in the p-doped material with respect to the n-doped material, electrons and holes generated in the vicinity of the p-n junction will drift to the n-doped and p-doped regions, respectively. Thus, the electron-hole pairs are collected systematically to provide positive charges at the p-doped region and negative charges at the n-doped region. The p-n or p-i-n junction forms the core of this type of photovoltaic device, which provides electromotive force that can power a device connected to the positive node at the p-doped region and the negative node at the n-doped region.
The majority of solar cells currently in production are based on silicon wafers with screen printed metal pastes as electrical contacts. Screen printing is attractive due to its simplicity in processing and high throughput capability; however, the high contact resistance, high paste cost, shadowing from wide conductive lines, high temperature processing, and mechanical yield loss are disadvantages that have not been overcome even after thirty plus years of research and development.
For advanced and experimental high efficiency solar cells in laboratories, vacuum based metallization processes may be used with an inevitable higher cost and low throughput.
Very recently, metallization with a plated copper grid has been reported. However, the plated copper can easily diffuse into the silicon solar cells and damage the solar cells performance. To prevent this detrimental effect, diffusion barriers, such as nickel silicide and/or nickel, have been employed. Nickel silicide may provide good contact resistance with silicon and at the same time, to a certain degree, prevent copper diffusion. A nickel layer may be used to improve adhesion or diffusion properties. However, even with these diffusion barriers, the solar cell performance may still degrade at a certain elevated temperature or after a certain long time of operation.
In addition, copper metallization is typically achieved utilizing electroplating. The electroplated Ni layer has to be coalescent to form a continuous Ni silicide layer. Therefore, the Ni layer that is formed by electroplating is generally a thick layer. Furthermore, the plated Ni layer can have variations in its thickness. As such, when the plated Ni is fully converted into Ni silicide during the silicidation process, a comparatively thick Ni silicide or a Ni silicide layer with comparatively large thickness variations forms. When the plated Ni is only partially converted into Ni silicide by controlling the silicidation process, for example the temperature, the thickness of Ni silicide can have large variation when the solar cell substrate has variations in its properties, resulting in both no silicide and thick Ni silicide at certain different locations. This thick Ni silicide can be thicker than the p-n junction in the solar cell and, as a result, the formation of a thick Ni silicide may damage the cell.
Therefore, a copper metalized solar cell structure with good contact resistance between metal and semiconductor substrate and further improved solar cell lifetime is needed.
Moreover, a method of making a copper metalized solar cell with a uniform metal silicide layer is needed. Such a method is particularly needed for solar cells with a semiconductor substrate having large variations in its properties.